Thin film y-junction circulator having significant shunt capacitance associated with each center conductor



FICANT Nov. 3, 197.0

Y THIN F Y-J R TOR HAVING .ISIGNI CIATED WITH EACH UCTOR R. H. KNERUNC'I'ION CIRCULA S U T CAPACI NCE ASSO C ER COND 2 Sheets-Sheet 1 FiledFeb. 5, 1969 GYROMAGNETIC MAT ERIAL FIG.

INVENTOR By R. H. KNERR I ATTORNEY Nov. 3, 1970 R. H. KNERR 3,538,459

THIN FILM Y-JUNCTION CIRCULATOR HAVING s1 FICANT v SHUNT CAPACITANCEASSOCIATED WITH E CENTER CONDUCTOR Filed Feb. 5, 1969 Z-Sheets-Sheet 2FIG. 4

United States Patent U.S. Cl. 333-11 7 Claims ABSTRACT OF THE DISCLOSUREA new form of Y-junction circulator adapted to photolithographicconstruction techniques. Integral forms of split conductors and shuntresonating capacitors reduce the overall size of the lumped elementcirculator and extend its range of applicability into higher regions ofthe microwave spectrum.

BACKGROUND OF THE INVENTION The present invention relates to a new formof Y-junction circulator adapted to photolithographic constructiontechniques. Integral forms of split conductors and shunt resonatingcapacitors reduce the overall size of the lumped element circulator andextend its range of applicability into higher regions of the microwavespectrum.

A common form of the Y-junction circulator consists of a ferrite coreand a distributed constant resonator or center conductor. The dimensionsof the center conductor are inversely proportional to the operatingfrequency of the circulator. With a disk-shaped resonator, for example,the diameter of the center conductor must be approximately equal toone-half the wavelength of the exciting signal, and if the frequency ofthe signal is halved, the diameter of the resonator must be doubled.

The prior art form of lumped element circulator, on the other hand, doesnot rely on dimensional resonance. In this form, which has been treatedanalytically in articles by Deutsch and Wieser, IEEE Transactions onMagnetics, vol. 2, No. 3, September 1966, pp. 278-82, and Konishi, IEEETransactions on Microwave Theory and Techniques, vol. 13, No. 6 November1965, pp. 852-64, and disclosed in patents to Roberts: 3,286,201 andKonishi: 3,335,374, the resonator includes a ferrite disk core and threetuned circuits, each containing a conductor and a discrete capacitiveelement. The conductors are radially disposed on the top surface of thecore, with one end grounded in a manner providing three-fold symmetry.When connected to sources of high frequency energy, these conductorsexhibit inductive reactances; therefore, the discrete capacitors areadded to each circuit to achieve the resonance necessary for fluxinducement in the core.

As is the case wtih the distributed constant form, the size of thelumped element circulator is substantially determined by its operatingfrequency, since the size of the discrete lumped inductive andcapacitive elements decreases with decreasing wavelength. Though bothcir culator forms are frequency dependent, over a range of frequenciesextending well into the gigahertz region the lumped element circulatoris more compact than a distributed constant circulator operating at thesame frequency.

However, those this size advantage in favor of the lumped element formexists up at least through X-band, in the present state of the art, itis nearly impossible to build discrete, non-integrated lumped elementswhich ice operate at frequencies much above two gigahertz. Therefore, atfrequencies above two gigahertz, the distributed constant form, in spiteof its greater size, is normally used.

At the present time, extensive eflort is being made in the art tocombine, make smaller and reduce the cost of microwave components. It isparticularly desirable, therefore, to achieve a practical lumped elementcirculator in the range at and above L-band which is more compact thanthe dimensionally resonant, distributed constant form.

SUMMARY OF THE INVENTION The present invention concerns a new form oflumped element circulator in which the discrete reactances of the lumpedelement approach are combined into an integrated member analogous to thecenter conductor of the distributed constant form. The resultingintegrated lumped element circulator, designed to be produced byphotolithographic techniques, is reduced in size over both of the priorart forms, and offers lower cost, more opportunity for integration, anda range of operation extending well into the gigahertz region of themicrowave spectrum.

Two versions of the integrated lumped element circulator are described.In a first embodiment, the necessary capacitance, added in shunt, isproduced between adjacent conductor segments, one of which is grounded,by means of interdigitating conducting fingers formed as part of eachconductor. In a second embodiment, a segment of each conductor ispositioned in close proximity to a ground ring, thereby providing adirect capacitive connection to ground. Both versions are intended to beproduced photolithographically.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammaticrepresentation of one embodiment of the present invention;

FIG. 2 is a secional view taken along the line 2-2 in FIG. 1;

FIG. 3 is a perspective view of a segment of the circulator shown inFIG. 1;

FIG. 4 is a diagrammatic representation of a second embodiment of thepresent invention;

FIG. 5 is a sectional view taken along the line 5-5 in FIG. 4; and

FIG. 6 is a perspective view of a portion of the circulator shown inFIG. 4.

DETAILED DESCRIPTION Referring to FIGS. 1-3, the circulator componentsinclude three conductors 11, 12 and 13, arranged in rotational symmetryon the upper face of substrate 20. The substrate is composed of agyromagnetic material, commonly a ferrite or a compositealumina-ferrite. The middle segment of each conductor is divided into atleast two split conductors, and the insulated crossing points of thesesplit conductors are arranged as shown in FIG. 1 to induce a moreuniform RF magnetic field inside the gyromagnetic material. As isdiscussed above, the con 'ductors exhibit inductive reactance at theoperating fre quency, and series or shunt capacitors must be added toeach circuit to achieve resonance.

In the first embodiment, shunt capacitors are composed of sets ofconducting fingers 21 through 26 extending out from the outer portionsof the conductors and proportioned so that fingers formed on facingsides of adjacent conductors interdigitate but do not touch. These setsof interdigitating, separated fingers produce capacitance between thetwo conductor segments of which they are part.

3 For example, finger set 23 forms a capacitor between the adjacentportions of conductors 11 and 13. The amount of capacitance isdetermined by the number of fingers, their dimensions, and the spacingbetween them.

Probes extending through eyelets 14, 15 and 16 connect one end of eachconductor to ground plane 17 located beneath substrate 20. As FIG. 1shows, the conductors are arranged so that their grounded ends are 120degrees apart. Each ungrounded end is thus flanked by two grounded ends,and, since finger sets are present in every sextant, each ungrounded endis connected to ground through two symmetrically arranged capacitors.

Circulators built according to the teachings of this embodiment possessseveral advantages. First, since the capacitive and inductive reactancesare part of the same member, they are produced simultaneously, incontrast to the discrete lumped element approach of Konishi and others.Second, since the capacitors are symmetrically connected from each inputport to ground, they provide a maximum uniformity of port-to-portcharacteristics. Third, with the use of photolithographic techniques,this approach should be feasible up to X-band whereas the discretelumped element approach using an assemblage of discrete elements is notbelieved to be feasible for operation above L-band. In the gigahertzrange, circulators built as herein disclosed are a half to a full orderof magnitude smaller than distributed constant circulators operating atthe same frequency.

Circulators of the present type have been tested at L-band: theinsertion loss is less than 1 db, maximum isolation is greater than dband the 20 db bandwidth is 2 percent. The device tested has not beenoptimized with respect to bandwidth and insertion loss so that furtherdevelopment can be expected to improve on this performance. Photoetchedseries resonant circuits could be added to further broadband thecirculator. The interdigital capacitors in this device lie within acircle of 0.5 inch diameter whereas an L-band distributed constantcirculator has a diameter of several inches. A similar circulatordesigned for operation at 4 gHz. has a 0.125 inch diameter. At this sizethe circulator appears compatible with encapsulated semiconductordevices.

An alternative embodiment of the present invention is pictured in FIGS.46. In this form ground ring 39 is contained in an annular groove 38sunk into the upper face of substrate 40. Ground plane 37 is locatedbeneath substrate 40 and is connected to ground ring 39 by probesextending through eyelets 34, and 36. The three conductors 31, 3-2 and33- are formed without the finger extensions appearing in the firstembodiment. They are positioned in rotational symmetry as before, withone end of each acting as a port and the other suitably connected toground ring 39.

Shunt resonating capacitance is included in this configuration at thepoints where the conductors pass over ground ring 39. Since ground ring39 is sunk into the upper face of substrate 40, the segments 31a, 32aand 33a of the conductors lying above ground ring 39 may be brought asclose as desired to it by properly setting the depth of groove 38. Eachof these conductor segments forms one face of an air gap capacitor, andthe underlying arc of ground ring 39 forms the other. The value of thecapacitors may be regulated by varying the dimensions of the facingplanes and the distance between them. This embodiment shares the cost,size and range of operation advantages of the first embodiment, and ittoo provides complete symmetry of the potential distribution.

It should be noted that other modifications of this embodiment arepossible. Specifically, the sunken groove 38 may be eliminated if groundring 39 is laid directly on the upper face of substrate 40 and adielectric layer of air, silicon dioxide or other suitable material ismaintained between the ring and the overlying conductor segments. Abetter alternative, and one particularly amenable to photolithographictechniques, recesses the ground ring and the entire conductor patterninto the upper portion of the gyromagnetic substrate and then providescapacitance at the crossovers through the use of an inserted dielectricmaterial as mentioned above. This embodiment is achieved through the useof one additional p'hotolithographic mask which delineates the patternto be etched into the substrate, and it provides excellent physicalprotection for the delicate crossover points.

It should further be noted that in both embodiments a second ferritedisk is normally, though not always, positioned above the areacontaining the split conductors, and that a second ground plane may bepositioned on top of the structure to achieve compatibility with otherportions of the circuit. When a second ground plane is present, eyelets34, 35 and 36 are shifted out from under the overlying conductorsegments so that ground connections may be conveniently made to bothground planes.

Other similar modifications will occur to those skilled in the art whichdo not depart from the spirit and scope of the present invention.

I claim:

1. A Y-junction circulator comprising a lower ground plane having acentral axis,

three coplanar conductors, each having one end grounded, radiallyarranged in rotational symmetry about the central axis above the groundplane so that their midpoints lie on the axis, their middle portionsoverlap, and the grounded end of a first conductor lies between theungrounded ends of the second and third conductors,

a member of gyromagnetic material located between the ground plane andthe conductors,

and means for symmetrically producing a significant shunt capacitanceassociated with each conductor, said means comprising at least onearcuate section of conducting material attached to and extending outwardfrom both sides of the grounded end of each conductor in a planeparallel to the plane of the conductors, the dimensions of the sectionbeing chosen so that a portion of the conducting material comes intoclose proximity with the ungrounded ends of the adjacent conductors.

2. A circulator as defined in claim- 1 including a second member ofgyromagnetic material positioned above the plane of the conductorscovering at least the overlapping middle portions of said conductors.

3. A circulator as defined in claim 1 including an upper ground planepositioned above the plane of the conductors.

4. A Y-junction circular comprising a lower ground plane having acentral axis,

three coplanar conductors, each having one end grounded, radiallyarranged in rotational symmetry about the central axis above the groundplane so that their midpoints lie on the axis, their middle portionsoverlap, and the grounded end of a first conductor lies between theungrounded ends of the second and third conductors,

a member of gyromagnetic material located between the ground plane andthe conductors,

and means for symmetrically producing a significant shunt interdigitalcapacitance associated with each.

conductor, said means comprising a plurality of conducting fingers lyingsubstantially in the plane of the conductors attached to and extendingoutward from both sides of each conductor, along the outer portions ofeach conductor, the dimensions of each finger and the spacing along theconductors between the fingers being adjusted so that the fingersextending from both sides of the grounded outer portion of a firstconductor and the fingers extending from the facing sides of theadjacent ungrounded outer conductor portions interdigitate but do nottouch.

5. A Y-junction circulator comprising a lower ground plane having acentral axis,

three coplanar conductors, each having one end grounded, radiallyarranged 120 degrees apart about the central axis above the ground planeso that their midpoints lie on the axis, their middle portions overlap,and the grounded end of a first conductor lies between the ungroundedends of the second and third conductors,

a member of gyromagnetic material located between the ground plane andthe conductors,

and means for symmetrically producing a significant shunt capacitanceassociated with each conductor, said means comprising an annular groundring positioned about the central axis between the gyromagnetic memberand the plane of the conductors,

said ring being connected to the grounded end of each conductor andhaving an outer radius smaller than the distance from the midpoint ofeach conductor to its ungrounded end so that a segment of each conductoradjacent to its ungrounded end passes over an arcuate portion of theground ring, and

means for maintaining a particular value of capacitance between theground ring and the overlying segment of each conductor.

6. A circulator as described in claim 5 wherein said means formaintaining a particular value of capaictance between the ground ringand the overlying segment of each conductor comprises.

an annular groove included in the upper face of the gyromagneticmaterial and containing the ground ring, the depth of the groove beingadjusted so that the perpendicular distance between the segment of eachconductor lying above the exposed ground ring and the ground ringproduces the particular capacitance. 7. A circulator as defined in claim5 wherein said means for maintaining a particular value of capacitancebetween the ground ring and the overlying segment of each conductorcomprises a layer of dielectric material of predetermined thicknesspositioned between the ring and the overlying segment.

References Cited UNITED STATES PATENTS 3,334,318 8/1967 Nakahara et al.3331.1 3,335,374 8/1967 Konishi 333-1.1

PAUL L. GENSLER, Primary Examiner U.S. Cl. X.R. 333-84

